The cascode is a well-known configuration with numerous applications, including avoiding the Miller effect, increasing voltage gain, and current buffering. For a cascode configuration, a common-source stage (or other transconductance stage) is coupled to a common-gate transistor. The common-gate transistor is often referred to as a “cascode transistor”. According to one usage of the word “cascode”, the transconductance stage (e.g. common-source transistor or differential pair) and the common-gate transistor coupled together is referred to as a “cascode”. According to another usage of the word “cascode”, which is the usage employed in this document, the “cascode” does not necessarily include the transconductance stage. Instead, to refer inclusively to the transconductance stage and a cascode transistor (and possibly other circuitry) the term “cascode amplifier”, “cascode OTA”, or the like, is used in this document.
Examples of cascode configurations include the telescopic cascode configuration and the folded cascode configuration.
FIG. 1A illustrates cascode amplifier 101, which is arranged in a telescopic cascode configuration. Transistor M1 is arranged in a common-source configuration, and provides current Icasc_in based on voltage Vin. Also, cascode 107 includes cascode transistor M0, which is arranged to provide current Icasc_out from current Icascode_in. Cascode transistor M0 provides a non-inverting voltage gain, and provides a current gain of substantially one. Cascode transistor M0 is arranged to receive cascode bias voltage Vcascode_bias at its gate.
FIG. 1B shows folded cascode OTA 102. Voltage Vin may be single-ended or differential. Gm stage 111 provides current Igm_out from voltage Vin. Folded cascode 103 provides current Icascode_out from current Igm_out. Current source IC1 provides a DC bias current. As with cascode transistor M0, cascode transistor M2 provides a non-inverting voltage gain, and provides a current gain of substantially one.
FIG. 1C illustrates folded cascode OTA 104 and active load 106. Folded cascode OTA 104 is similar to folded cascode OTA 102 of FIG. 1B, except that current Igm_out is differential. As with cascode transistors M0 and M2, cascode transistor M3 provides current Icascode_out from current Icascode_in, and provides a non-inverting voltage gain, and a current gain of substantially one. Transistors M5 and M6 are arranged to provide DC bias currents. Further, node N0 is a high-impedance node since it is coupled to the output of cascode transistor M3 and active load 106. Current Icascode_out is converted into voltage Vcasc_out at node N0 due to the impedance at node N0.